Hey there South.Texas
I'm not sure what the differences may be. I know that around '82, MEC changed their 600s so that some of the dies were plastic (wad guide too)... maybe that is the difference.
I recently purchased a 12 gauge sizemaster and some 16 gauge sizemaster dies... and swapped them out with no problems. (It was after that swap that I realized that there were SM77 and SM82).
If you really need to know, here's MECs website. Call them up. They are VERY helpful. (and I used to call them quite often)
I've got 4 single stage presses (old 600s, versamecs, and a new 600) that have ALL had the dies swapped off and on. The parts always seem to fit... http://mecreloaders.com/main.asp
I hope that helps.
PS.... IF you get a good deal on a Sizemaster (I wouldn't worry about SM77 vs. SM82) .